The present invention relates to a magnetic recording medium and a substrate therefor, and more particularly, to a magnetic recording medium comprising a non-magnetic undercoat layer having a smooth surface, a high strength and a lower light transmittance and a low electric resistance even at a small carbon black content, and a substrate for the magnetic recording medium.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
A reduction in the thickness of a magnetic recording layer is described. Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), " . . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness, . . . ".
Development of a thinner film for a magnetic recording layer has caused some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Synthetic Technology" (1987), published by the Publishing Department of Technology Information Center, " . . . the surface roughness of a hardened magnetic coating film depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic coating film is formed on the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved."
Secondly, there has been caused a problem in the strength of a base film with a tendency of the reduction in the thickness of the base film in response to the demand for a thinner magnetic coating film. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, " . . . Higher recording density is a large problem assigned t the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded, . . . "
The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black fine particles or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.
However, addition of a large amount of non-magnetic particles such as carbon black fine particles impairs not only the enhancement of the magnetic recording density but also the development of a thinner recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black fine particles which are added to a magnetic recording layer.
Consequently, it has been strongly demanded to provide a magnetic recording medium capable of exhibiting a low light transmittance even when the amount of carbon black fine particles added to a magnetic recording layer thereof is reduced as low as possible. From this viewpoint, it has been strongly required to essentially improve properties of a substrate therefor.
Further, in order to reduce not only the above-mentioned light transmittance of the magnetic recording medium but also an electric resistance thereof, there has been hitherto proposed a method of adding carbon black fine particles to the magnetic recording layer.
The conventional magnetic recording medium to which carbon black fine particles are added, is described in detail below.
When a magnetic recording medium has a high surface resistivity, an electrostatic charge thereon tends to be increased, so that cut chips of the magnetic recording medium and dirt or dusts are attached onto the surface of the magnetic recording medium upon production or use thereof, and as a result, such a problem that the number of "drop-out" becomes increased, is caused.
In order to decrease the surface resistivity of the magnetic recording medium to about 10.sup.8 .OMEGA./sq, a conductive compound such as carbon black fine particles has been ordinarily added to the magnetic recording layer in an amount of not less than about 5 parts by weight based on 100 parts of magnetic particles contained in the magnetic recording layer.
However, when the amount of carbon black fine particles added or the like which cannot contribute to improvement in magnetic properties of the magnetic recording layer, is increased, the magnetic recording medium has been deteriorated in signal recording property as described above, resulting in inhibiting the magnetic recording layer from being thinned.
With the reduction in thickness of the magnetic recording layer or the base film therefor, various attempts have been conducted in order to improve properties of a substrate on which the magnetic recording layer is formed, thereby enhancing a surface smoothness and a strength of the magnetic recording medium. In this regard, there has been proposed a method of forming on a base film, at least one undercoat layer composed of a binder resin and non-magnetic particles dispersed in the binder resin and containing iron as a main component, such as acicular hematite particles or acicular iron oxide hydroxide particles (hereinafter referred to merely as "non-magnetic undercoat layer"), and such a method has been already put into practice (Japanese Patent Publication (KOKOKU) No. 6-93297(1994), Japanese Patent Application Laid-open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993), 5-182177(1993), 5-347017(1993) and 6-60362(1994), and the like).
It is known to use as the non-magnetic particles for non-magnetic undercoat layer, non-magnetic particles which surfaces are coated with hydroxides of aluminum, oxides of aluminum, hydroxides of silicon, oxides of silicon or a mixture thereof in order to improve the dispersibility thereof in vehicles, thereby enhancing a surface smoothness and a strength of the obtained non-magnetic substrate (Japanese Patent Nos. 2,571,350 and 2,582,051, and Japanese Patent Application Laid-open (KOKAI) Nos. 6-60362(1994), 9-22524(1997) and 9-27117(1997) or the like).
Also, it is known that in order to decrease an amount of carbon black fine particles added to the magnetic recording layer and reduce a light transmittance of the magnetic recording medium, blackish brown acicular hematite particles or blackish brown acicular ferric oxide hydroxide particles are used as the non-magnetic particles for non-magnetic undercoat layer (Japanese Patent Application Laid-open (KOKAI) Nos. 7-66020(1995) and 8-259237(1996)).
Further, it is known that in order to reduce an electric resistance of the magnetic recording medium, a mixture of non-magnetic iron oxide particles and carbon black fine particles is used as the non-magnetic particles for non-magnetic undercoat layer (Japanese Patent Application Laid-open (KOKAI) Nos. 1-213822(1989), 1-300419(1989), 6-236542(1994) and 9-297911(1997) or the like).
With the reduction in thickness of not only the magnetic recording layer but also the base film, it has been most demanded to provide such a magnetic recording medium having a more smooth surface, a high strength, and a lower light transmittance and a lower electric resistance even when the amount of carbon black fine particles in the magnetic recording layer is reduced as low as possible. However, at present, magnetic recording media capable of satisfying all of such requirements have not been obtained yet.
Namely, in the case of the above-mentioned conventional substrates produced by using as the non-magnetic particles for non-magnetic undercoat layer, acicular hematite particles, acicular iron oxide hydroxide particles, acicular hematite particles coated with hydroxides of aluminum or the like, or acicular iron oxide hydroxide particles coated with hydroxides of aluminum or the like, although surface smoothness and high strength of the substrate using these particles as non-magnetic particles are satisfied, it is difficult to reduce the light transmittance due to dark red or yellowish brown color of the non-magnetic particles. In addition, the non-magnetic substrate exhibit a surface resistivity as large as not less than about 10.sup.13 .OMEGA./sq.
Further, in the case of the above-mentioned substrate using as non-magnetic particles for non-magnetic undercoat layer, blackish brown acicular hematite particles or blackish brown acicular iron oxide hydroxide particles, the obtained non-magnetic undercoat layer can show a higher degree of blackness as compared to those using the dark red acicular hematite particles or the yellowish brown acicular iron oxide hydroxide particles, so that it becomes possible to reduce a light transmittance of the substrate. However, the reduction of light transmittance is still insufficient. In addition, the surface resistivity of the non-magnetic substrate is as large as about 10.sup.12 .OMEGA./sq.
In the case of the non-magnetic particles described in the above-mentioned Japanese Patent Application Laid-open (KOKAI) Nos. 1-213822(1989), 1-300419(1989) and 9-297911(1997), there have been used the mixture of non-magnetic iron oxide particles and carbon black particles in which the carbon black particles are added in an amount of not less than 25 parts by weight based on 100 parts by weight of the non-magnetic iron oxide particles. Therefore, due to the fact that the carbon black particles which show the largest degree of blackness among various black pigments, are used in such a large amount, it is possible to obtain a non-magnetic substrate having a low light transmittance and a low electric resistance. However, it has been difficult to disperse the carbon black fine particles in vehicles, since the carbon black fine particles are fine particles having such a small average diameter as about 0.002 to about 0.05 .mu.m, a large specific surface area and a deteriorated solvent wettability, thereby failing to obtain a non-magnetic substrate having a smooth surface and a high strength. In addition, the carbon black fine particles have a bulk density as low as about 0.1 g/cm.sup.3 and, therefore, the carbon black fine particles are bulky particles, resulting in deteriorated handing property and workability. Further, with respect to the carbon black fine particles, many problems concerning safety or hygiene such as carcinogenesis have been pointed out.
Thus, as the amount of the carbon black fine particles added to the non-magnetic substrate is increased, the light transmittance of the obtained non-magnetic substrate tends to become small. However, when the carbon black fine particles are used in a large amount, it becomes more difficult to disperse the particles in vehicles, resulting in deteriorated workability. Further, the use of a large amount of the carbon black fine particles are disadvantageous in view of safety and hygiene.
In the above-mentioned Japanese Patent Application Laid-open (KOKAI) No. 6-236542(1994), there have been described such non-magnetic particles mixture of non-magnetic iron oxide particles and carbon black fine particles having a specific structure in which the carbon black fine particles are used in an amount of 1 to 17.6 parts by weight based on 100 parts by weight of the non-magnetic iron oxide particles. By using such specific carbon black fine particles having a high conductivity, the electric resistance of the obtained non-magnetic substrate can be reduced even at a small carbon black fine particles content. However, since the amount of carbon black fine particles used is small, it is difficult to reduce a light transmittance of the non-magnetic substrate.
In consequence, it has been strongly demanded to provide a magnetic recording medium and a non-magnetic substrate therefor which can exhibit a smooth surface, a high strength, and a lower light transmittance and a lower electric resistance even at a small carbon black content.
As a result of the present inventors' earnest studies, it has been found that by using as non-magnetic particles contained in a non-magnetic undercoat layer, non-magnetic acicular black iron-based composite particles comprising:
acicular hematite particles or acicular iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA1 a coating layer formed on the surface of the said acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA1 a carbon black coat formed on the said coating layer comprising said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular hematite particles or acicular iron oxide hydroxide particles, PA1 the obtained non-magnetic substrate can exhibit a more excellent smooth surface, a high strength, and a lower light transmittance and a lower electric resistance even at a small carbon black content. The present invention has been attained on the basis of this finding. PA1 a non-magnetic base film; PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and PA1 a magnetic coating film comprising a binder resin and magnetic particles, PA1 the said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and PA1 a magnetic coating film comprising a binder resin and magnetic particles, PA1 the said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and PA1 a magnetic coating film comprising a binder resin and magnetic particles, PA1 the said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and PA1 a magnetic coating film comprising a binder resin and magnetic particles, PA1 said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; and PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles, PA1 the said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; and PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles, PA1 the said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; and PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles, PA1 said non-magnetic acicular black iron-based composite particles comprising: PA1 a non-magnetic base film; and PA1 a non-magnetic undercoat layer formed on the said non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles, PA1 the said non-magnetic acicular black iron-based composite particles comprising:
(1) organosilane compounds obtained from an alkoxysilane compounds, PA2 (2) polysiloxanes or modified polysiloxanes, and PA2 (3) fluoroalkyl organosilane compounds obtained from a fluoroalkylsilane compounds; and PA2 acicular hematite particles or acicular iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coating layer formed on the surface of the said acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising the said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular hematite particles or acicular iron oxide hydroxide particles. PA2 acicular hematite particles or acicular iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coat formed on at least a part of the surface of the said acicular hematite particles or acicular iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon; PA2 a coating layer formed on the said coat formed on the surface of the said acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising the said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular hematite particles or acicular iron oxide hydroxide particles. PA2 acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coating layer formed on the surface of the said acicular manganese-containing hematite particle or acicular manganese-containing iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising the said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular hematite particles or acicular iron oxide hydroxide particles. PA2 acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coat formed on at least a part of the surface of the said acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon; PA2 a coating layer formed on the said coat formed on the surface of the said acicular manganese-containing hematite particle or acicular manganese-containing iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles. PA2 acicular hematite particles or acicular iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coating layer formed on the surface of the said acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising said organosilicon compound coated, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular hematite particles or acicular iron oxide hydroxide particles. PA2 acicular hematite particles or acicular iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coat formed on at least a part of the surface of the said acicular hematite particles or acicular iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon; PA2 a coating layer formed on the said coat formed on the surface of the said acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular hematite particles or acicular iron oxide hydroxide particles. PA2 acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coating layer formed on the surface of the said acicular manganese-containing hematite particle or acicular manganese-containing iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles. PA2 acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles having an average major axis diameter of 0.02 to 0.30 .mu.m; PA2 a coat formed on at least a part of the surface of the said acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon; PA2 a coating layer formed on the said coat formed on the surface of the said acicular manganese-containing hematite particle or acicular manganese-containing iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of: PA2 a carbon black coat formed on the said coating layer comprising said organosilicon compound, in an amount of 1 to 20 parts by weight based on 100 parts by weight of the said acicular manganese-containing hematite particles or acicular manganese-containing iron oxide hydroxide particles.